This invention relates generally to semiconductor fabrication equipment, and more particularly to steppers and scanners.
Steppers and scanners are types of semiconductor fabrication equipment used in photolithographic processing, such as aligning a mask over a wafer and exposing the pattern of the mask onto the wafer. A scanner typically uses a mirror system with a slit blocking part of the light coming from the light source. The size of the slit is smaller than the wafer, so the light beam scans across the wafer. Whereas scanning is generally performed on a per-wafer basis, a stepper is utilized on only a given part of the wafer at one time. A reticle is aligned and exposed, without scanning, and then is stepped to the next site and the process is repeated. Stepping generally allows more precise matching of larger-diameter wafers than scanners do.
Another type of semiconductor fabrication equipment combines the stepping and scanning process of steppers and scanners, and is known as step and scan aligners. At one position on the semiconductor wafer, a small-scale scanning process takes place, and then the reticle or mask is stepped to the next position, where the scanning process is repeated. As used herein, steppers, scanners, and step and scan aligners are generally encompassed under the term alignment and exposure equipment, which can include other types of specific semiconductor fabrication equipment besides steppers, scanners, and step and scan aligners. Examples of steppers, scanners, and step and scan aligners include those available from ASML Holding, N.V., of the Netherlands. Furthermore, unless otherwise and specifically noted, steppers, scanners, and step and scan aligners are used substantially interchangeably herein, such that reference to or description of one should be assumed to apply to other types of alignment and exposure equipment as well.
Scanners and steppers can generally achieve a degree of focus for a given exposure time. A process window, such as an exposure-defocus (ED) window, maps the ranges within which acceptable lithographic quality occurs. The process window for a given feature shows the ranges of exposure dose and depth of focus (DOF) that permit acceptable lithographic quality for the semiconductor device being fabricated.
For example, FIG. 1 shows a graph 100 of a typical ED process window for a given semiconductor pattern feature. The y-axis 102 indicates exposure dose of the light source being used, whereas the x-axis 104 indicates DOF. The line 106 maps exposure dose versus DOF at one end of the tolerance range for the critical dimension (CD) of the pattern feature, whereas the line 108 maps exposure dose versus DOF at the other end of the tolerance range for the CD of the feature.
The area 110 enclosed by the lines 106 and 108 is the ED process window for the pattern feature, indicating the ranges of both DOF and exposure dose that permit acceptable lithographic quality of the feature. Any DOF-exposure dose pair that maps within the area 110 permits acceptable lithographic quality of the pattern feature. As indicated by the area 110, the process window is typically indicated as a rectangle, but this is not always the case, nor is it necessary.
A difficulty with using any process window, however, is that the focus of the stepper or scanner generally changes, or drifts, over time. One of the causes of focus drift over time is the heat subjected on the lens from the light source. As heat builds up on the lens, the focus of the lens can sufficiently change to affect the process window for the critical dimensions of the semiconductor device being fabricated. To compensate for this, most lens have a heating factor that can be adjusted to stabilize focus.
However, the focus and image sensors of most steppers and scanners cannot monitor focus drift in real time, but rather can only indicate the focus after processing of a given number of wafers has been accomplished. This makes it difficult to tune the heating factor to compensate for the heating of the lens, and results in the inability to compensate for undesirable focus drift. In turn, the undesirable focus drift effectively reduces the process window for the lithographic processing, and decreases CD uniformity.
FIG. 2 shows a graph 200 depicting undesirable focus drift and the general inability of current steppers and scanners to counter it. The y-axis 202 measures focus drift, whereas the x-axis 204 measures time, or more specifically the number of wafers that have been so far processed on a given stepper or scanner. Focus drift increases to an unacceptable level throughout initial processing of the wafers by the stepper or scanner, as indicated by the line 206. At the point 208, processing has been completed of the current lot of wafers, and the mask or reticle of the stepper or scanner is changed.
This change allows a cool-down period, indicated by the line 210, in which the stepper or scanner is not being used, such that focus drift is effectively decreasing. At the point 212, processing continues again. Focus drift then decreases, as indicated by the line 214, because the heating factor of the lens can be modified before starting processing again. The heating factor of the lens cannot be modified while processing is occurring, however.
Therefore, there is a need for lens heating compensation that overcomes the disadvantages associated with such compensation as found in the prior art. Specifically, there is a need for compensating for lens heating that reduces undesirable focus drift. Such a reduction in focus drift should prevent process window reduction, and maintain CD uniformity. Furthermore, there is a need for compensating for lens heating that can be accomplished substantially in real time. For these and other reasons, there is a need for the present invention.
The invention relates to a variable transmission focal mask to compensate lens heating. A semiconductor fabrication alignment and exposure equipment includes an exposure and alignment unit, a variable transmission mask, and a stage. The unit has a light source and a lens. The mask is under the lens, and at least indirectly measures focus. The mask further can adjust the focus in real time in response to determining that the focus is out of specification. A wafer is placed on the stage for exposure to the light source through a mask or a reticle. The variable transmission mask preferably normally has a substantially high transmission of light rating that can be adjusted downward to adjust the focus. For example, the mask can be a liquid crystal display (LCD) that can be darkened to so reduce its transmission of light rating.
Embodiments of the invention provide for advantages over the prior art. The variable transmission mask normally passes through a high percentage of the light from the light source of the exposure and alignment, such as 90%. However, as the lens of the exposure and alignment unit heats up, the focus of the mask on the semiconductor wafer drifts. This is detected by the variable transmission mask, and in response the mask darkens, or otherwise reduces the transmission of light therethrough, maintaining the focus. Thus, reducing the transmission of light through the variable transmission mask compensates for the heating of the lens, and the focus drift that this heating causes. Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings.